Cellular glutathione peroxidase (GPX1) is the first identified and a major selenium (Se)-dependent enzyme, its activities in tissues have been used to determine dietary Se requirements of humans and animals. Because we still do not know the metabolic functions of GPX1, these allowances for Se are not well founded physiologically. We propose to use both GPX1 overexpressing and knockout mouse models to determine whether GPX1 is a major selenoperoxidase against various forms of oxidative stress in vivo, and whether the antioxidative function of GPX1 is affected by dietary vitamin E and/or other selenoperoxidases. Our long-term objective is to elucidate the physiological role of GPX1 gene expression in Se nutrition. We will conduct nine major experiments to determine the role of GPX1 in protecting against acute, chronic, and metabolic oxidative stress and the interaction of GPX1 with vitamin E and other selenoperoxidases in this regard. The acute oxidative stress will be generated by administration of lethal and sublethal doses of pro-oxidant compounds, the chronic oxidative stress by feeding dietary polyunsaturated fatty acids, and the metabolic oxidative stress by aging and pregnancy. We will determine the physiological or pathological responses of whole body which include survival time, clinic signs, tissue lesions, growth and reproductive performance, and general health. We will measure the biochemical changes of various tissues which include antioxidant status, peroxidation of protein and lipid, damage of DNA, and mRNA and activity expression of GPX1 and several other Se-dependent or independent enzymes. We will also test the predisposed oxidative status of isolated hepatocytes and thymocytes. Our key approach will be to compare the differences in these responsess between the GPX1 overexpressing. GPX1 knockout, and their respective control mice, and to compare the effect of various dietary levels of vitamin E and Se on the role of GPX1. Our study will illustrate unequivocally the antioxidative role of GPX1 and its interaction with vitamin E and other selenoperoxidases in vivo at the gene expression level. These results can be applied to accurately assess dietary Se needs and facilitate other usage of Se for optimal health.